In molecular beam epitaxy (MBE), elemental source beams with cosinusoidal intensity profiles are directed across a vacuum space onto a heated substrate crystal 10. The substrate 10 is heated to provide sufficient energy for surface diffusion and incorporation of the species. The elemental source beams originate from separate ovens, often called effusion cells 12. The heated elements vaporize and exit the effusion cell 12 through one end, go past an open shutter 14, and travel through the growth chamber 16 until reaching the substrate 10. Elements may be switched on and off using the shutter 14 in front of each effusion cell 12. FIG. 1 shows a schematic diagram of the growth chamber 16 of a prior art MBE machine. To compensate for the natural inhomogeneity of the beam intensity profiles and the necessarily nonsymmetric arrangement of sources within the growth chamber 16, the beams are not centered on the circular substrate 10, rather they are focused onto an intermediate point along the substrate radius. The substrate 10 is then rotated to achieve compositional uniformity.
To ensure uniformity, the rotation rate is chosen such that less than one monolayer is grown in the time it takes for a single substrate revolution. For a growth rate of 2 A/s, compositional uniformity, both in the growth direction and radially across the substrate 10, requires rotation speeds of 60 rpm or more. However, practical considerations such as sample mounting, impurity control, and low maintenance growth procedures dictate a considerably lower substrate rotation speed, typically 2-10 rpm.
A superlattice 18, SL, is a one-dimensional periodic structure consisting of ultrathin layers 44, with its period less than the electron mean free path. A schematic of a superlattice 18 is shown in FIG. 2. These ultra-thin layers 44 differ, in a prescribed manner, by their energy gaps. As the dimensions of the period become comparable to the electron wavelength, wave properties of the charge carriers become important. The energy profile of the superlattice 18 consists of alternating energy barriers as shown in FIG. 3. Classically, an electron with energy Ef, approaching an energy barrier with an energy below the barrier energy, Eb, would be reflected, analogous to a baseball rebounding off a concrete wall or to an electromagnetic wave at the end of an open-circuited transmission line. Quantum mechanics, however, allows that as the physical dimensions of the barrier decrease toward the wavelength of the particle, there is an increasing probability that the particles will be transmitted instead of reflected. Thus under certain conditions an electron can pass through the barrier even with energy below the barrier potential. This classically-forbidden phenomena is called tunneling. Due to this phenomena, some desirable aspects of superlattices are higher mobility and, lower electron scattering rates, translating into reduced parasitic resistance, and higher current gain in vertical devices such as heterojunction bipolar transistors (HBTs) and hot electron transistors.
Superlattices 18 are obtained by a periodic variation of composition during epitaxial growth. Heretofore, in this field, superlattice structures 18 have been formed using mechanical shuttering in molecular beam epitaxy (MBE) growth. To introduce the periodic variation of composition, the elemental beam fluxes across the substrate 10 are varied by mechanically shuttering the effusion cells 12. This shuttering action, however, can cause growth transients and "shutter fatigue". Growth transients occur due to the pressure build up of the vaporized element in a closed effusion cell 12. Once the cell is opened again, a burst of atoms impinge upon the substrate 10, increasing the growth rate until the pressure has stabilized. "Shutter fatigue" occurs when the shutters 14 jam or don't open or close properly. Shutter 14 fatigue is especially common in superlattice 18 growth due to the excessive number of transitions the shutters 14 must perform to fulfill the ultra-thin and multiple layer 44 requirements.
The materials currently of interest in superlattice growth are well-known semiconductors and their alloys; for examples, Ge, Si, Ge--Si alloys, III-V compounds and their alloys, II-VI compounds and their alloys, etc. InGaAs--InAlAs pseudomorphic and lattice-matched heterojunctions on InP substrates are of particular interest for use in high-performance transistor structures such as high electron mobility transistors (HEMTs), and resonant tunneling transistors (RTTs) because of the superior electron transport properties of InGaAs and the large conduction band offset (0.52 eV obtained for the InGaAs/InAlAs heterojunction).